<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="vue.js"></script>
</head>
<body>
  <div id="app">
    <input type="text" v-model="firstName">
    <input type="text" v-model="lastName">
    =>
    {{ fullName }}
  </div>
  <script>
    let vm = new Vue({
      el:"#app",
      data:{
        firstName:"",
        lastName:""
      },
      computed:{
        fullName(){
          return this.firstName+"."+this.lastName
        }
      }
    })
  </script>
</body>
</html>